The present invention relates to an optical scanning apparatus for use in a multicolor image forming apparatus such as a color copier, a color printer, a color facsimile or the like.
Recently, a digital copier and a laser-beam printer are commercially practical as multicolor image forming apparatuses forming color images. Either of the apparatuses forms a color image in the following manner. Four photosensitive drums are disposed in a conveying direction of output paper. After these photosensitive drums are uniformly charged by respective chargers, they are simultaneously exposed to respective laser beams generated by respective optical scanning apparatuses, thereby forming four electrostatic latent images. These four electrostatic latent images are developed by respective development units using respective toners of different colors i.e., yellow, magenta, cyan and black, and successively transferred so as to overlap one another, thereby forming a color image.
As described above, such a tandem-type multicolor image forming apparatus generally uses individually positioned optical scanning apparatuses corresponding to respective colors, which are yellow, magenta, cyan and black. However, the individually positioned optical scanning apparatuses corresponding to the respective colors for formation of a color image are likely to cause color misregistration.
Causes of the color misregistration are as follows.
For example, in cases where polygon mirror scanner motors are used as light deflectors, even if rotational speed of the motors is controlled respectively based on a clock having the same frequency as those of the other motors, a deviance of printing width occurs because inertial weight, a coil winding state, rotator""s balance and the like vary from motor to motor. Moreover, since in each of the optical scanning apparatuses installed is different optical system and different housing, incident laser beams for the respective colors bring about relative shifts to the rest such as printing inclination and scanning curvature. Furthermore, in cases where semiconductor lasers are used as beam generators, wavelength variation of each of the lasers resulting from atmospheric temperature variation takes place. For instance, in the event that black printing is performed first and successively four-color printing is performed, the wavelength of the semiconductor laser for black printing becomes different in the second printing due to an atmospheric temperature rise. As a result, a diffraction index of the laser for black differs from those of the lasers for the other three colors, thus causing color misregistration.
Conventionally, in order to solve the foregoing problems, a detector reads an actual print, and based on its detected result, a modification is made to a dot clock to correct a deviance of printing width. Moreover, an actuator is attached to a reflection mirror installed in each of the optical scanning apparatuses, and each reflection mirror is moved in accordance with the amount of color misregistration, thereby correcting relative scanning inclination and relative scanning curvature.
However, the aforementioned technique requires actual printing for every correction of color misregistration. In particular cases where such a correction is made with respect to the temperature rise within the apparatus, since actual printing is performed for every certain temperature rise, toner is consumed in large quantity, whereby the increases in running costs and in waste toner take place. Moreover, in the event that a waste toner box that is small in capacity is integrally formed with the development unit, it needs to be replaced together with the development unit before the lifetime of the development unit comes to an end. On the other hand, a sufficient capacity of the waste toner box with respect to the lifetime of the development unit entails upsizing of the multicolor image forming apparatus. Moreover, additional costs such as the cost of detector for measuring the color misregistration amount based on the actual print and the cost of actuators for moving the reflection mirrors of the respective optical scanning apparatuses cannot be avoided. The foregoing conventional technique is thus an impedimental to the reduction in size and cost of the multicolor image forming apparatus.
In order to solve the foregoing problems, a technique of simultaneously exposing the photosensitive drums to the respective laser beams for the respective colors which are outputted from a single optical scanning apparatus comprised of a single deflector, a single optical system and a single housing has been proposed.
However, since a relationship between a focal distance and a pitch of the photosensitive drums was undefined in this technique, the focal distance called for a redundant length for avoiding printing errors. Consequently, even such a technique has entailed the upsizing of the optical scanning apparatus and the increase in cost.
The present invention addresses the problems discussed above, and aims to provide an optical scanning apparatus capable of preventing color misregistration and realizing size reduction and cost reduction of the apparatus.
The optical scanning apparatus of the present invention comprises:
(a) a single deflector disposed in a direction perpendicular to a deflection plane for deflecting and scanning a plurality of bundled incident laser beams;
(b) a single conversion optical system disposed on optical paths of the laser beams scanned by the deflector for converting the bundle of laser beams moving at a constant angular velocity into a bundle of laser beams moving at a constant linear velocity;
(c) a plurality of reflection mirrors disposed on the optical paths of the respective laser beams emitted from the conversion optical system for separating the bundle of laser beams into laser beams corresponding to respective colors;
(d) a plurality of photosensitive members disposed at equal intervals for forming respective electrostatic latent images by means of the laser beams corresponding to the respective colors, wherein one of the photosensitive members is disposed on a plane parallel to an area defined by the laser beams traveling between the deflector and the conversion optical system; and
(e) a correction optical system for correcting curvatures of field with respect to the respective laser beams and leading the laser beams to the photosensitive members, respectively, the correction optical system comprising correcting devices respectively disposed on the optical paths of the laser beams separated by the reflection mirrors.
This structure in which one of the photosensitive members is disposed on the above-described plane realizes the reduction in size and cost of the apparatus.